🚀 go-pugleaf

In article <C1It67.5E5@world.std.com>, DPierce@world.std.com (Richard D Pierce) writes:
> In article <1993Jan27.000515.19195@parc.xerox.com> tapscott@parc.xerox.com (Peter Tapscott) writes:
> 
> In demonstrating how much power you can derive from an amplifier based on
> a 12 volt rail can supply, Peter Tapscott says:
>>
>>You have to be cautious about accepting wattage ratings
>>from car amps. The manufacturers frequently exceed the laws of physics
>>with their ratings.
>>
>>P = 16/R for a 12 volt DC supply. For an 8 ohm speaker, the max
>>output is only about 2 watts. Figure 4 watts for a 4 ohm speaker.
>>
>> . . .
>>
>>I would be suspicious of claims of driving loads much below 4 ohms, except
>>for very expensive audiophile car amps, like Orion. 
>>
> 
> [...]
> It's now a trivial excercise to design and mass produce car audio
> electronics that have output amplifiers that run of supply rails that are
> much in excess of 12 volts. 
> [...]

Yes.  And it is even more trivial to build an amp that gives about 18 watts RMS
(usually claimed to be a 25W or 30W amp at an un-specified distortion level :-)
without a DC-to-DC converter.  It's called a "bridging" amp.  

Ignoring the "13.6 volts" that a car really supplies, and ignoring the voltage
drop through the output devices:  At 0 input the + and - outputs for the
channel are both at +6 volts, so no current flows through the speaker.  When
the input waveform is "all the way positive", the + output terminal goes to +12
and the other to 0; when the input waveform is "all the way negative", the +
output goes to 0 and the - to 12 volts.  This gives a 24V peak-to-peak swing
across the speaker.  This works out to about 18 watts.  Practically, these
designs usually achieve 12 or 14 watts RMS.  

> It's cheap, it's easy, it's common, and it's what you missed in your
> discussion.

What he said! :-)

	--- Jamie Hanrahan, Kernel Mode Systems, San Diego CA
Internet:  jeh@cmkrnl.com, or hanrahan@eisner.decus.org  Uucp: uunet!cmkrnl!jeh